Polyurethane-based universal household adhesive

ABSTRACT

The invention relates to the use of a substantially clear and at least largely solventless, aqueous, one-component polyurethane dispersion based on the reaction products of 
     a polyol mixture consisting completely or partly of polypropylene glycol, 
     a mixture of polyfunctional isocyanates consisting completely or partly of α,α,α&#39;,α&#39;-tetramethyl xylene diisocyanate (TMXDI), 
     a functional component capable of salt formation in aqueous solution and 
     optionally a chain-extending agent 
     as a universal household adhesive and to a process for its production in the absence of inert solvents.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the use of special, substantially clear,aqueous and at least largely solventless polyurethane dispersions asuniversal household adhesives and to a process for their production.

Universal household adhesives, also known as multipurpose adhesives, areused for bonding a number of substrates encountered in the home (paper,cardboard, photographs, fabrics, leather, felt, bast, cork, films,metals, such as aluminium and iron, china, ceramics, glass, wood,various plastics, including for example polystyrene foams). Theadhesives in question are expected to produce an adequate adhesiveeffect on these various substrates which differ chemically andphysically in their surface structure and which are normally subjectedto a special surface treatment before bonding.

Compared with the large variety of classes and types of adhesive used inindustry and workshops, there are only a few substances which arecapable of meeting the stringent demands imposed on the universality ofa multipurpose household adhesive. Among those substances, onlypolyvinyl acetate and its copolymers are widely used--normally insolution or, for glueing wood, in the form of a dispersion.

The demand for universality represents a particularly difficultselection criterion for an adhesive. Ultimately, it means that theadhesive molecules must show equally high affinity for polar and apolarinterfaces. Accordingly, the statement that a certain substance issuitable as an adhesive does not indicate to the expert whether it canalso be used as a universal household adhesive.

In addition to the universality requirement, there has also recentlybeen a demand for solventless, physiologically safe, clear, aqueousformulations of neutral odor in the field of universal householdadhesives. However, these formulations are intended at the same time tolead to adhesives of which the dried films in turn have a certainresistance to water. In addition, these water-based adhesives are alsointended to be able to bond substrates that are difficult to bond, suchas plastics. They are also intended to have a long storage life.

This requirement profile could not be fully satisfied either on thebasis of the binders hitherto preferred for universal adhesives, namelypolyvinyl acetate and vinyl acetate copolymers, or by such alternativesas nitrocellulose. Although polyvinyl acetate can be produced withoutsolvents in the form of aqueous dispersions, the dispersions obtainedare not transparent, but milky white. They show good performanceproperties when used, for example, as wood glue. The acrylates andstyrene acrylates widely used as dispersion adhesives are also not knownon the market in the form of transparent household adhesives with theproperties mentioned.

It is known that specially selected embodiments of the aqueouspolyurethane dispersions known for decades are suitable as multipurposeuniversal adhesives. Polyurethane dispersions consist of adducts ofpolyfunctional isocyanates (isocyanate component) with polyfunctional OHcompounds (polyol component) which contain co-condensed units capable ofsalt formation in aqueous solution. It has surprisingly been found thatpolyurethane dispersions in which the polyol component is based onpolypropylene glycol and the isocyanate component is based ontetramethyl xylene diisocyanate, are also suitable as universalhousehold adhesives and provide good adhesion values. It has alsosurprisingly been found that dispersions of the type in question can beproduced without using inert solvents.

2. Discussion of Related Art

DE-OS 36 30 045 broadly describes an adhesive based on an aqueousdispersion of polyurethanes containing incorporated carboxylate and/orsulfate groups of which the basic diisocyanate component consists of amixture of at least two (cyclo)aliphatic diisocyanates. The DE-OSmentions numerous polyols, numerous isocyanate compounds and numeroussalt-forming components as modifying agents. It also mentions thechain-extending agents typically used in the field in question. Theadhesives are said to be suitable for bonding any substrates.

The broad disclosure of DE 36 30 045 in regard to the polyol componentencompasses both polyurethane dispersions, in which polyesters are usedas the OH-functional component, and also those in which polyethers areused. However, polyurethane dispersions based on polyurethanessynthesized from OH-functional polyesters are not suitable as universalhousehold adhesives because they undergo hydrolysis in storage and hencedo not have the required stability in storage.

Although DE 15 95 602 also mentions polymerization products of propyleneoxide, for example, as a possible polyol, there is no reference to thefact that polyols such as these are a suitable basis for polyurethanedispersions for universal household adhesives having the requirementprofile mentioned above.

Although universal household adhesives based on polyurethane dispersionswhich have the stated requirement profile are already known from DE 3827 378, the dispersions in question are not dispersions in which thepolyurethanes are formed by reaction of polypropylene glycol withisocyanates.

Although polymerization products of propylene oxide are mentioned amongmany other polyethers in DE 38 27 378 and in the corresponding EP 354471, they are only mentioned as an addition to the described polyolmixture consisting of polytetrahydrofuran. At the same time, it ispointed out in both patents that polyurethane dispersions based onpolyethylene oxide and/or polypropylene oxide as the OH-functionalcomponent are also unsuitable as multipurpose adhesives because theyshow poor adhesion to plastic surfaces and, accordingly, do not satisfythe universality requirement. This is consistent with the statement inDE-OS 17 69 387 that polyether diols which have been prepared fromoxides, such as ethylene oxide or propylene oxide, are unsuitable aspolyol component for the production of polyurethane dispersions intendedfor the bonding of plasticized PVC plastics.

Among many other suitable isocyanates, EP 354 471 also mentionstetramethyl xylene diisocyanate, although there is no reference to thefact that the combination of this isocyanate with a polyol componentbased on polypropylene glycol leads to polyurethane dispersions which,when used as multipurpose adhesives, do show good adhesion, for exampleto plastic surfaces. In addition, there is no reference to the factthat, where this combination is used, the dispersion can be preparedwithout inert solvents. Instead, the acetone process is preferred inboth the cited patents.

Accordingly, the problem addressed by the present invention was to showthat special, aqueous, transparent polyurethane dispersions based on apolyol component which, in turn, is based on polypropylene glycolsatisfy the complex and partly conflicting requirements mentioned abovein regard to universal household adhesives. These requirements alsoinclude very good stability to hydrolysis and high adhesive strength.The invention also sought to provide a process by which it would bepossible, in contrast to the prior art, to produce polyurethanedispersions suitable as domestic multipurpose adhesives without havingto use inert solvents, so that the dispersions could even be producedwithout any residual solvent content whatever.

DESCRIPTION OF THE INVENTION

Other than in the operating examples, or where otherwise indicated, allnumber expressing quantities of ingredients or reaction conditions usedherein are to be understood as modified in all instances by the term"about."

The present invention relates to the use of a substantially clear and atleast largely solventless, aqueous, one-component polyurethanedispersion based on the reaction products of

a polyol mixture consisting completely or partly of polypropyleneglycol,

a mixture of polyfunctional isocyanates consisting completely or partlyof α,α,α',α'-tetramethyl xylene diisocyanate (TMXDI),

a functional component capable of salt formation in aqueous solution and

optionally a chain-extending agent

as a universal household adhesive.

The present invention also relates to a process for the production ofthe polyurethane dispersions suitable as universal household adhesives.

The polyurethanes forming the basis of the polyurethane dispersions usedin accordance with the invention are based on a polyol mixtureconsisting completely or partly of polypropylene glycol in which thepolypropylene glycol content, based on the polyol mixture, should be noless than 30% by weight and preferably no less than 50% by weight.Mixtures containing more than 70% by weight polypropylene glycol arepreferred. A polyol mixture consisting entirely of technical gradepropylene glycol is particularly suitable. This variant is preferredinter alia because, on the one hand, polyurethane dispersions preparedfrom the polyol mixture perform well in performance tests and because,on the other hand, the production process is simplified to the extentthat one step--the incorporation of one or more polyols--is saved.Isotactic polypropylene glycol is also suitable.

Polypropylene glycol belongs to a class of compounds well known to theexpert, cf. Kirk-Othmer, Encyclopedia of Chemical Technology, 3rdEdition, Vol. 18, John Wiley & Sons, N.Y. 1982, pages 633-645. In theoryand even in practice, polypropylene glycols are adducts of propyleneoxide started with a molecule containing at least one active hydrogen.Such molecules are alcohols, polyalcohols, amines or even water.Glycerol, propylene glycol, ethylene glycol and trimethylol propane areindustrially significant. Suitable amines are, for example ethylenediamine, diethylene triamine and toluene diamine. Other suitable startermolecules and suitable, commercially available polypropylene glycols canalso be found in the cited literature reference. Polypropylene glycolshaving a molecular weight in the range from 200 to 5,000 and, moreparticularly, in the range from 300 to 3,000 have proved to beparticularly favorable. Suitable polypropylene glycols may also containsmall quantities, more particularly up to 10 mol. %, based on the eductmonomers, of other alkyl oxide units, preferably ethylene oxide, in thepolymer chain. These alkyl oxide units may be present in statisticaldistribution and/or in blocks in the polymer.

One particularly advantageous embodiment of the invention enablescompletely solventless universal household adhesives to be produced.Completely solventless in the present context means that there are evenno residual contents of solvents. This can be ensured by producing thepolyurethane dispersions without using solvents. According to theinvention, therefore, a solventless polyurethane dispersion preparedwithout using solvents is preferably used as the universal householdadhesive.

In addition, up to 70% by weight, but preferably less, of thepolypropylene glycols on which the polyurethane dispersions used inaccordance with the invention are based may be replaced by other polyolstypically used in preparations of the type in question. A general rulein this regard is that these other polyols must contain at least tworeactive hydrogen atoms and should be substantially linear with amolecular weight in the range from 300 to 20,000 and preferably in therange from 400 to 6,000. Preferred other polyols are polyesters,polyacetals, polycarbonates, polyethers, polythioethers, polyamidesand/or polyester amides containing on average 2 to at most 4 hydroxylgroups.

In the context of the invention, polycarbonates are understood to bepolyesters which, theoretically, can be prepared by esterification ofcarbonic acid with dihydric or polyhydric alcohols and which bear ahydroxyl group at either end of the chain. The alcohols and, ultimately,the polycarbonate diols preferably have an aliphatic structure. Suitablepolyhydric alcohols may be, for example, trihydric, such as glycerol forexample. However, dihydric alcohols are preferred, particularly if theycontain no less than four and no more than ten carbon atoms. Althoughcyclic and branched alcohols are suitable, linear alcohols arepreferred. The hydroxyl groups may be arranged adjacent, for example inthe 1,2-position, or even isolated. OH-terminated diols are preferred.Suitable polycarbonate diols have a molecular weight in the range from500 to 8,000 and preferably in the range from 800 to 2,500.

Suitable polyethers are, for example, the polymerization products ofethylene oxide, butylene oxide and copolymerization or graftpolymerization products thereof and the polyethers obtained bycondensation of polyhydric alcohols or mixtures thereof and thepolyethers obtained by alkoxylation of polyhydric alcohols, amines,polyamines and aminoalcohols. Other suitable polyethers are thepolytetrahydrofurans described in the cited EP 354 471 andethylene-glycol-terminated polypropylene glycols.

Suitable polyacetals are, for example, the compounds obtainable fromglycols, such as diethylene glycol, triethylene glycol, hexanediol andformaldehyde. Suitable polyacetals may also be prepared bypolymerization of cyclic acetals.

Among the polythioethers, the condensates of thiodiglycol on its ownand/or with other glycols, dicarboxylic acids, formaldehyde,aminocarboxylic acids or aminoalcohols are mentioned in particular.Depending on the co-components, the products are polythioethers,polythio mixed ethers, polythioether esters, polythioether ester amides.Polyhydroxyl compounds such as these may also be used in alkylated formor in admixture with alkylating agents.

The polyesters, polyesteramides and polyamides include the predominantlylinear condensates obtained from polybasic, saturated and unsaturatedcarboxylic acids or their anhydrides and polyhydric, saturated andunsaturated alcohols, aminoalcohols, diamines, polyamines and mixturesthereof and also, for example, polyterephthalates. Polyesters oflactones, for example caprolactone, or of hydroxycarboxylic acids mayalso be used. The polyesters may contain terminal hydroxyl or carboxylgroups. Relatively high molecular weight polymers or condensates suchas, for example, polyethers, polyacetals, polyoxymethylenes may also be(co-)used as alcohol component in their synthesis.

Polyhydroxyl compounds already containing urethane or urea groups andoptionally modified natural polyols, such as castor oil, may also beused. Basically, polyhydroxyl compounds containing basic nitrogen atomssuch as, for example, polyalkoxylated primary amines or polyesters orpolythioethers containing alkyl diethanolamine in co-condensed form mayalso be used. Polyols obtainable by complete or partial ring opening ofepoxidized triglycerides with primary or secondary hydroxyl compounds,for example the reaction product of epoxidized soybean oil withmethanol, are also suitable. Copolymers of the polyhydroxyl compoundsmentioned are also suitable, as are their analogs preferably terminatedby amino or sulfide groups.

According to the invention, the balance to 100% by weight of polyolmixtures consisting partly of polypropylene glycol preferably consistsof polytetrahydrofurans, polyethylene glycols, polyacetals,polycarbonates and/or polyesters containing on average 2 to at most 4 OHgroups.

The mixture of polyfunctional isocyanates on which the polyurethanedispersions is based consists completely or partly ofα,α,α',α'-tetramethyl xylene diisocyanate (TMXDI). The meta- and/orpara-isomeric form is particularly suitable. Only with a minimum contentof TMXDI in the isocyanate mixture is it possible to obtain apolyurethane dispersion with a polypropylene-glycol-based polyolcomponent which gives good adhesion values when used as a universalhousehold adhesive. Accordingly, preferably at least 20% by weight and,better yet, at least 35% by weight of the isocyanate mixture consists ofTMXDI. As a general rule, the adhesion values mentioned are better, thehigher the content of TMXDI in the isocyanate mixture. Accordingly,mixtures of which half or more, for example two thirds or threequarters, contain TMXDI are preferred.

Suitable additional polyisocyanates, i.e. making up the balance to 100%by weight, are any polyfunctional aromatic and aliphatic isocyanatessuch as, for example, 1,5-naphthylene diisocyanate, 4,4'-diphenylmethanediisocyanate, 4,4'-diphenyl dimethyl methane diisocyanate, di- andtetraalkyl diphenyl methane diisocyanate, 4,4'-dibenzyl diisocyanate,1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, the isomers oftolylene diisocyanate, optionally in admixture,1-methyl-2,4-diisocyanatocyclohexane, 1,6-diisocyanato-2,2,4-trimethylhexane, 1,6-diisocyanato-2,4,4 -trimethyl hexane,1-isocyanatomethyl-3-isocyanatomethyl-3-isocyanato-1,5,5-trimethylcyclohexane, chlorinated and brominated diisocyanates,phosphorus-containing diisocyanates, 4,4'-diisocyanatophenylperfluoroethane, tetramethoxy butane-1,4-diisocyanate,butane-1,4-diisocyanate, hexane-1,6-diisocyanate, dicyclohexyl methanediisocyanate, cyclohexane-1,4-diisocyanate, ethylene diisocyanate,phthalic acid-bis-isocyanatoethyl ester, also polyisocyanates containingreactive halogen atoms, such as 1-chloromethylphenyl-2,4-diisocyanate,1-bromomethylphenyl-2,6-diisocyanate,3,3-bis-chloromethylether-4,4'-diphenyl diisocyanate. Sulfur-containingpolyisocyanates are obtained, for example, by reaction of 2 molhexamethylene diisocyanate with 1 mol thiodiglycol or dihydroxydihexylsulfide. Other important diisocyanates are trimethyl hexamethylenediisocyanate, 1,4-diisocyanatobutane, 1,2-diisocyanatododecane and dimerfatty acid diisocyanate. Of interest are partly masked polyisocyanateswhich provide for the formation of self-crosslinking polyurethanes, forexample dimeric tolylene diisocyanate, or polyisocyanates partlyreacted, for example, with phenols, tertiary butanol, phthalimide,caprolactam.

In one preferred embodiment, the isocyanate component at least partlycontains dimer fatty acid isocyanate. Dimer fatty acid is a mixture ofpredominantly C₃₆ dicarboxylic acids which is obtained by thermal orcatalytic dimerization of unsaturated C₁₈ monocarboxylic acids, such asoleic acid, tall oil fatty acid or linoleic acid. Dimer fatty acids havelong been known to the expert and are commercially available. Dimerfatty acid can be reacted to dimer fatty acid isocyanates. Dimer fattyacid diisocyanate is preferred for the purposes of the invention.Technical grade dimer fatty acid, gliisocyanate contains on average atleast two and less than three isocyanate groups per molecule dimer fattyacid.

The isocyanates mentioned above may be used individually or inadmixture. It is preferred to use--in particular cyclic orbranched--aliphatic diisocyanates, more especially isophoronediisocyanate.

The suitable polyfunctional isocyanates preferably contain on average 2to at most 4 NCO groups. The quantities of polyol mixture and of themixture of polyfunctional isocyanates are selected so that a certainratio of NCO-reactive groups to NCO groups is present. The isocyanatecomponent is preferably not present in a substoichiometric quantity but,at the same time, does not exceed two and a half times the percentage ofNCO-reactive groups. A ratio of 1:1.05 to 1:2 is particularly favorable.The preferred range--and also the optimal range in regard to thesubsequent performance results--is from 1:1.1 to 1:1.6.

The chain extending agents containing reactive hydrogen atoms include

the usual saturated and unsaturated glycols, such as ethylene glycol orcondensates of ethylene glycol, butane-1,3-diol, butane-1,4-diol,butenediol, propane-1,2-diol, propane-1,3-diol, neopentyl glycol,hexanediol, bis-hydroxymethyl cyclohexane, dioxyethoxy hydroquinone,terephthalic acid-bis-glycol ester, succinic acid di-2-hydroxyethylamide, succinic acid di-N-methyl-(2-hydroxyethyl)-amide,1,4-di-(2-hydroxymethylmercapto)-2,3,5,6-tetrachlorobenzene,2-methylenepropane-1,3-diol, 2-methylpropane-1,3-diol;

aliphatic, cycloaliphatic and aromatic diamines, such asethylenediamine, hexamethylenediamine, 1,4-cyclohexylenediamine,piperazine, N-methyl propylenediamine, diaminodiphenylsulfone,diaminodiphenyl ether, diaminodiphenyl dimethylmethane,2,4-diamino-6-phenyl triazine, isophorone diamine, dimer fatty aciddiamine; (not included are diamines having unwanted properties whichcould endanger health, such as for example hydrazine, diaminodiphenylmethane or the isomers of phenylenediamine; also carbohydrazides orhydrazides of dicarboxylic acids)

aminoalcohols, such as ethanolamine, propanolamine, butanolamine,N-methyl ethanolamine, N-methyl isopropanolamine;

aliphatic, cycloaliphatic, aromatic and heterocyclic mono- anddiaminocarboxylic acids, such as glycine, 1- and 2-alanine,6-aminocaproic acid, 4-aminobutyric acid, the isomeric mono- anddiaminobenzoic acids, the isomeric mono- and diaminonaphthoic acids;

water.

It is emphasized that it is not possible in the context of the inventionstrictly to distinguish between compounds containing reactive hydrogenatoms with a molecular weight in the range from 300 to 20,000 and theso-called "chain extending agents" because the boundaries between thetwo classes of compounds are not clearly defined. Compounds which do notconsist of several monomer units, but have a molecular weight above 300,such as for example 3,3'-dibromo-4,4'-diaminodiphenyl methane, areclassed as chain-extending agents as is pentaethylene glycol although,on the basis of its composition, the latter is actually a polyetherdiol.

Special chain-extending agents containing at least one basic nitrogenatom are, for example, mono-, bis- or polyalkoxylated aliphatic,cycloaliphatic, aromatic or heterocyclic primary amines, such asN-methyl diethanolamine, N-ethyl diethanolamine, N-propyldiethanolamine, N-isopropyl diethanolamine, N-butyl diethanolamine,N-isobutyl diethanolamine, N-oleyl diethanolamine, N-stearyldiethanolamine, ethoxylated coconut oil fatty amine, N-allyldiethanolamine, N-methyl diisopropanolamine, N-ethyl diisopropanolamine,N-propyl diisopropanolamine, N-butyl diisopropanolamine, C-cyclohexyldiisopropanolamine, N,N-diethoxylaniline, N,N-diethoxyl toluidine,N,N-diethoxyl-1-aminopyridine, N,N'-diethoxyl piperazine,dimethyl-bis-ethoxylhydrazine,N,N'-bis-(2-hydroxyethyl)-N,N'-diethylhexahydrop-phenylenediamine,N-12-hydroxyethyl piperazine, polyalkoxylated amines, such aspropoxylated methyl diethanolamine, also such compounds asN-methyl-N,N-bis-3-aminopropylamine, N-(3-aminopropyl)-N,N'-dimethylethylenediamine, N-(3-aminopropyl)-N-methyl ethanolamine,N,N'-bis-(3-aminopropyl)-N,N'-dimethyl ethylenediamine,N,N'-bis-(3-aminopropyl)-piperazine, N-(2 -aminoethyl) -piperazine, N,N'-bisoxyethyl propylenediamine, 2,6-diaminopyridine,diethanolaminoacetamide, diethanolamidopropionamide,N,N-bisoxyethylphenyl thiosemicarbazide, N,N-bis-oxyethylmethylsemicarbazide, p,p'-bis-aminomethyl dibenzyl methylamine,2,6-diaminopyridine,2-dimethylaminomethyl-2-methylpropane1,3-diol.

Chain-extending agents containing halogen atoms or R-SO₂ O groupscapable of quaternization are, for example, glycerol-1-chlorohydrin,glycerol monotosylate, pentaerythritol-bis-benzenesulfonate, glycerolmonomethanesulfonate, adducts of diethanolamine and chloromethylatedaromatic isocyanates or aliphatic haloisocyanates, such asN,N-bis-hydroxyethyl-N'-m-chloromethyl phenylurea,N-hydroxyethyl-N'-chlorohexyl urea, glycerol monochloroethyl urethane,bromoacetyl dipropylene triamine, chloroacetic acid diethanolamide.Short-chain isocyanate-reactive diamines and/or dihydroxy compounds arepreferred chain-extending agents.

In addition, the polyurethanes on which the polyurethane dispersionsused in accordance with the invention are based contain co-condensedcarboxylic acids, sulfonic acids or ammonium compounds containing 1 to 2isocyanate-reactive groups as functional components capable of saltformation in aqueous solution. These functional components may bedihydroxy or even diamino compounds containing an ionizable carboxylicacid, sulfonic acid or ammonium group. These compounds may either beused as such or they may be prepared in situ. To introduce compoundsbearing ionizable carboxylic acid groups into the polyurethane, theexpert may add to the polyols special dihydroxycarboxylic acids whichare only capable of secondary reactions between the carboxyl groups andthe isocyanate groups to a limited extent, if at all. A preferreddihydroxycarboxylic acid is, for example, dimethylol propionic acid.

To introduce sulfonic acid groups capable of salt formation, adiaminosulfonic acid may be added to the polyols. Examples are2,4-diaminobenzenesulfonic acid and also theN-(ω-aminoalkane)-ω'-aminoalkanesulfonic acids described in DE 20 35732.

To introduce ammonium groups capable of salt formation into the polymer,it is also possible in accordance with DE 15 95 602 cited above tomodify the polyurethane prepolymer with an aliphatic and aromaticdiamine in such a way that the chains are terminated by primary aminogroups which may then be converted into quaternary ammonium compounds orinto amine salts with standard alkylating agents.

According to the invention, it is preferred to make the polyurethaneprepolymers to be used soluble or redispersible in water by means ofcarboxylic acid or sulfonic acid groups because polyurethane dispersionscontaining anionic modifiers such as these can be removed under alkalineconditions, i.e. adhesives of the type in question can be removed fromcertain substrates, for example from fabrics, under washing conditions.

The polymers are present in salt form in the polyurethane dispersionsused in accordance with the invention. In the case of the preferredpolymers modified with carboxylic acids or sulfonic acids, alkali metalsalts, ammonia or amines, i.e. primary, secondary or tertiary amines,are present as counterions. In the cationically modified products, acidanions, for example chloride, sulfate or the anions of organiccarboxylic acids, are present as counterions. The groups capable of saltformation may therefore be completely or partly neutralized by thecounterions. An excess of neutralizing agent is also possible.

To obtain substantially clear, i.e. opaque to waterclear, polyurethanedispersions, it is important to maintain a certain ratio between thecomponent capable of salt formation and the other polyuretharie-formingcomponents. Thus, it is best to use the component capable of saltformation in quantities of from 10 to 9,D mol. %, preferably inquantities of from 20 to 80 mol. % anid, more preferably, in quantitiesof from 30 to 70 mol. %, based on the polyol mixture plus thechain-extending agent optionally present. In addition, the transparencydepends; on the degree of neutralization. The expert is able through afew preliminary tests to determine beyond what quantity of modifiercapable of ion formation or beyond what quantity of neutralizing agentan adequate degree of transparency is obtained. In general, as little ofthese components as possible will be used because they can adverselyaffect the water resistance and resistance to humidity of the adhesivefilm if used in excessive quantities.

The polyurethane dispersions according to the invention are understoodto be two-phase water/polyurethane systems which preferably encompasscolloidal systems and sols having particle diameters of up to about 200nm. These systems are preferably optically opaque to transparent.

The solids content of the adhesive solutions according to the inventionmay be varied within wide limits. Solids contents of from 20 to 70% byweight and preferably from 30 to 50% by weight have proved effective inpractice.

As already mentioned in the discussion of the chain-extending agentssuitable for use in accordance with the invention, compounds havinghealth-damaging or health-impairing properties are undesirable, all themore so as the present invention is concerned with a universal adhesivewhich, presumably, is also going to be used by people whose health ismore at risk, such as children, the elderly, the sick, pregnant women,etc. In one particularly preferred embodiment, therefore, not only arephysiologically harmful substances, such as hydrazine, not includedamong the chain-extending agents, the other components of the adhesiveare also selected for their physiological harmlessness. The danger tohealth from free isocyanates or unreacted NCO groups of the polymers orprepolymers, which is often discussed in connection with polyurethaneadhesives, does not arise in the case of the present invention becausethe polyurethanes are dispersed in water and, as any expert knows,isocyanate groups react off with water so that the polyurethanedispersions according to the invention can be guaranteed to contain noreactive NCO groups.

To produce the polyurethanes particularly suitable for the purposes ofthe invention, the polyols and an excess of diisocyanate are reacted toform an isocyanate-terminated polymer, suitable reaction conditions andreaction times and also temperatures being variable according to theparticular isocyanate. The expert knows that the reactivity of theconstituents to be reacted necessitates a corresponding equilibriumbetween reaction velocity and unwanted secondary reactions which lead todiscoloration and a reduction in molecular weight. The reaction istypically carried out with stirring over approximately 1 to 6 hours atapproximately 50° C. to approximately 120° C.

Suitable production processes for polyurethane dispersions aredescribed, for example, in D. Dieterich, Angew. Makromol. Chem. 98, page133 (1981) Ullmann, Encyklopadie der technischen Chemie, 4th Edition,Vol. 19, Verlag Chemie, Weinheim/BergstraSe 1974, pages 311-313;Houben-Weyl, Methoden der organischen Chemie, Vol. E 20, Part 1-3, pages1659-1663 and pages 1671-1681, and in Journal Waterborne Coating, Aug.1984, pages 2 el Em. The expert so inclined can also find particulars ofthe relevant patent literature in the secondary literature referencescited in these Articles. As known from the already cited EP 354 471, thepolyurethane dispersions hitherto suitable as universal householdadhesives are preferably produced by the so-called acetone process.Additions of low-boiling solvents, such as acetone for example, arenecessary inter alia for reducing the viscosity of the prepolymer andhence to ensure that it can be handled so that dispersion is actuallypossible. The disadvantage of such production processes, taking intoaccount the need for solventless universal adhesives, is that atechnically involved distillation step had to be carried out after thedispersion step to remove at least most of the low-boiling solvent. Thisinvolves an additional process step which not only complicates theprocess, it also adds to the cost of the product, not least because theacetone preferably used cannot be readily returned to the processbecause anhydrous acetone is preferably used. So far as the expert isconcerned, this is inevitably linked inter alia with the question ofwhether and to what extent a residual solvent content is acceptablebecause this determines the cost of the process. However, this doesconflict with the need for a solventless product. This requirement isactually dictated by consumers in the public debate on solvent residues,for example in household products. Accordingly, there is a need forprocesses which, on the one hand, lead to the desired products withoutadversely affecting their desired properties and, on the other hand, canbe carried out without any solvent at all, because residual solventcontents can also be avoided in this way.

The hitherto known polyurethane dispersions suitable as universaldomestic adhesives cannot be produced by a variant of the solventlessdispersion process because even the starting products have an excessiveviscosity. On the other hand, known polyurethane dispersions which canbe prepared by solventless dispersion processes are not suitable asuniversal domestic adhesives. Polyurethane dispersions of this type are,for example, those which contain polypropylene glycol as the keyconstituent of the polyol component. As can be seen from ComparisonExample 1 (which corresponds to Comparison Example 1 of EP 354 471) ofthe Examples of the present application, polyurethane dispersions showinadequate adhesion to substrate surfaces, particularly plasticsurfaces, which makes them unsuitable as universal domestic adhesives.In general, it is well known to the expert that polyurethane adhesivesbased on polypropylene glycol have distinctly poorer adhesion values andtensile shear strengths on almost all surfaces than, for example,polyurethane adhesives based on polyester polyols.

According to the invention, it has been found that a substantially clearand at least substantially solventless aqueous one-componentpolyurethane dispersion based on the reaction products

of a polyol mixture consisting completely or partly of polypropyleneglycol,

a mixture of polyfunctional isocyanates consisting completely or partlyof α,α,α',α'-tetramethyl xylene diisocyanate (TMXDI),

a functional component capable of salt formation in aqueous solution and

optionally a chain-extending agent,

can be produced by a solventless process. In other words, the reactionof the reactants mentioned to form the reaction products and thedispersion of the prepolymer phase can be carried out in the absence ofinert solvents. To this end, the reactants described above are normallymixed at room temperature. In general, the reaction may be carried outin standard tank reactors. The reaction temperature is in the range fromabout 70° to 110° C. The reaction mixture may contain effectiveadditions of catalysts for polyurethane reactions. The reactants arenormally stirred until the desired NCO value is obtained. The reactionmay be carried out by the so-called one-reactor process and also by theso-called two-reactor process. In the first case, the polyurethaneprepolymers are dispersed with vigorous stirring while water containingthe quantity of base required for neutralization is introduced. On theother hand, however, the prepolymer phase can also be introduced intothe aqueous base solution and dispersed therein with vigorous stirring.In both cases, dispersion can take place at elevated temperatures andmay optionally be followed by stirring for one to two hours.

The invention described in the foregoing is illustrated by the followingExamples.

EXAMPLES General Production Procedure

The reaction components were mixed at room temperature and stirred in astandard tank reactor at temperatures of 70° to 110° C. until there isno further reduction in the NCO content. Dispersion took place withvigorous stirring while the quantity of base required for neutralizationwas introduced. The reaction mixture was then stirred for 1 to 2 hours.

EXAMPLE 1

0.5 mol dimethylolpropionic acid

0.1 mol polypropylene glycol, MW 400

0.1 mol polytetrahydrofuran KW 650

0.3 mol polypropylene glycol KW 1000

1.4 mol TMXDI

Solids content: 35%, viscosity: 3,500 mPas

Appearance: opaque-transparent.

EXAMPLE 2

0.55 mol dimethylolpropionic acid

0.15 mol polypropylene glycol, MW 400

0.25 mol polypropylene glycol MW 1,000

0.05 mol polypropylene glycol MW 2,000

0.44 mol IPDI

0.86 mol TMXDI

Solids content: 34.2%, viscosity: 3,000 mpas

Appearance: opaque-transparent.

COMPARISON EXAMPLES 1 AND 2 Acetone Process

The polyols are diluted with acetone or dissolved or dispersed therein.The component capable of ion formation is then added with stirring.Diisocyanate is then added at temperatures of 50° to 70° C. until thereis no further reduction in the NCO content. Water containing the basesrequired for neutralization is then added. Liquid tertiary amines mayalso be added to the prepolymer shortly before dispersion. After about30 minutes, water is added. After dispersion for at least 30 minutes,the acetone is distilled off, ultimately under a relatively high vacuumat temperatures of 55° to 60° C.

COMPARISON EXAMPLE 1

    ______________________________________                                        Glycerol-started polypropylene                                                                     100    pbw    0.09 mol                                   glycol, OH value 34, MW 3,500                                                 Oleochemical polyol corresponding to                                                               82     pbw    0.36 mol                                   DE-PS 37 04 350, OH value 160                                                 Dimethylolpropionic acid                                                                           2l.50  pbw    0.55 mol                                   Acetone              50     pbw    2.0  mol                                   Isophorone diisocyanate                                                                            98.05  pbw    1.45 mol                                   Sodium hydroxide     6.40   pbw    0.36 mol                                   Water, deionized     450    pbw    56.0 mol                                   ______________________________________                                    

COMPARISON EXAMPLE 2

0.50 mol dimethylolpropionic acid

0.20 mol polypropylene glycol, MW 400

0.30 mol polypropylene glycol MW 1,000

0.6 mol isophorone diisocyanate

0.7 mol hexane diisocyanate

Solids content: 34%, viscosity: 4,000 mPas

Appearance : opaque-transparent

Adhesive strength (tensile shear strength, DIN 53 254) in N/mm²

    ______________________________________                                                   Example Comparison Example                                                    1    2       1        2                                            ______________________________________                                        Wood*/wood*  8.4    7.8     4.5    7.0                                        Wood*/ABS    3.5    3.3     1.4    1.8                                        Wood*/Plexiglas                                                                            5.6    5.3     1.5    2.0                                        Wood*/Alu    7.5    7.6     4.9    6.5                                        ______________________________________                                         *Beech plywood                                                           

The test specimens (5×10 cm²) were bonded after slight roughening andfailed after 3 days at a rate of advance of 10 cm/mins.

In the above Examples, MW stands for average molecular weight (approx.values).

We claim:
 1. A universal household adhesive composition comprising a substantially clear and solvent-free, aqueous, one-component polyurethane dispersion containing the reaction products of(a) a polyol mixture comprising polypropylene glycol, (b) a mixture of polyfunctional isocyanates comprising α, α, α¹, α¹ -tetramethyl xylene diisocyanate, (c) a functional component capable of salt formation in aqueous solution, and (d) optionally, a chain-extending agent.
 2. A universal household adhesive composition as in claim 1 wherein said polypropylene glycol has a molecular weight of from about 200 to about 5,000.
 3. A universal household adhesive composition as in claim 1 wherein said polyol mixture contains at least about 30% by weight of polypropylene glycol.
 4. A universal household adhesive composition as in claim 3 wherein said polyol mixture further contains polytetrahydrofuran, polyethylene glycol, polyacetal, polycarbonate and polyester containing on average 2 to 4 OH groups.
 5. A universal household adhesive composition as in claim 1 wherein said mixture of polyfunctional isocyanates contains at least about 20% by weight of said α, α, α¹, α¹ -tetramethyl xylene diisocyanate.
 6. A universal household adhesive composition as in claim 5 wherein said mixture of polyfunctional isocyanates contains an aliphatic isocyanate containing on average 2 to 4 NCO groups.
 7. A universal household adhesive composition as in claim 1 wherein said polyol mixture and said mixture of polyfunctional isocyanates are present in a ration of NCO-reactive groups to NCO groups of from about 1:1 to about 1:2.5.
 8. A universal household adhesive composition as in claim 1 wherein said functional component capable of salt formation in aqueous solution is selected from the group consisting of a carboxylic acid, sulfonic acid, and ammonium compound containing 1 to 2 isocyanate-reactive groups.
 9. A universal household adhesive composition as in claim 8 wherein said functional component is in the form of an alkali metal salt, ammonium salt, or salt of a primary, secondary or tertiary amine.
 10. A universal household adhesive composition as in claim 8 wherein said carboxylic acid comprises dimethylol propionic acid.
 11. A universal household adhesive composition as in claim 1 wherein said functional component capable of salt formation is present in an amount of about 10 to about 90 mol. %, based on said polyol mixture plus any content of chain-extending agent.
 12. A universal household adhesive composition as in claim 1 containing a chain-extending agent selected from the group consisting of an isocyanate-reactive diamine and a dihydroxy compound.
 13. A universal household adhesive composition as in claim 1 wherein the clarity of said composition is controlled by the quantity present of said functional component capable of salt formation.
 14. A universal household adhesive composition as in claim 1 wherein the clarity of said composition is controlled by the quantity of neutralizing agent added for said functional component capable of salt formation.
 15. The process of producing a substantially clear and solvent-free, aqueous, one-component polyurethane dispersion comprising reacting(a) a polyol mixture comprising polypropylene glycol, (b) a mixture of polyfunctional isocyanates comprising α, α, α¹, α¹ -tetramethyl xylene diisocyanate, (c) a functional component capable of salt formation in aqueous solution, and (d) optionally, a chain-extending agent,in the absence of an inert solvent, and dispersing the reaction products in the absence of an inert solvent.
 16. A process as in claim 15 wherein the reactants are mixed at room temperature, the mixture is stirred at a temperature of about 70° C. to about 110° C. until the desired NCO value has been obtained, and the reaction mixture is dispersed in water.
 17. A process as in claim 16 wherein said water contains a quantity of base material sufficient to neutralize the polyurethane prepolymers. 